Scintillator plates serve to convert X-ray or gamma radiation into visible light and are used in medical technology and for non-destructive material testing. Typically, scintillator plates include a substrate on which a scintillator layer is arranged. The material of a scintillator layer is suitable for absorbing high-energy photons of X-ray and gamma radiation and for re-emitting the energy of the high-energy photons in the form of a large number of low-energy photons, e.g., in the visible region (therefore referred to in the following as light photons). It is thereby possible to process the information of the X-ray or gamma radiation with common optical sensors. The two-dimensional detection of an image is possible when using a matrix of photodetectors, such as for example CCD sensors made of crystalline silicon or photodiode arrays made of amorphous silicon.
In order to achieve a high resolution in the image, as many as possible of the light photons generated by a single high-energy photon are received by a single photodetector of the matrix in order to obtain the information concerning the spatial correlation.
One of the scintillator materials used is caesium iodide (referred to below as CsI). A scintillator layer made of CsI is typically generated by vapor-deposition of CsI onto a substrate in a vacuum. The CsI tends, under suitable manufacturing conditions, to form columnar microstructures or needles, which are separate from one another and which grow upwardly away from the substrate. The microstructures typically have structure sizes in the lateral direction of the order of 10 μm. Due to the effect of total reflection, these microstructures are suitable for conducting a large part of the photons generated in the interior thereof along the microstructure, similarly to a light guide. Only light photons that meet the surface at too large an angle leave the respective microstructure and scatter in the wider surroundings.
If the layer thickness of the scintillator layer is selected to be larger, the microstructures tend to grow together and to come into contact with one another. This effect arises, e.g., in layer thicknesses from 300 μm upward. At contact sites of this type, the conditions, in part, no longer exist for total reflection, so that additional light photons are scattered out of the microstructures. Such layer thicknesses are used, e.g., when X-ray or gamma radiation with relatively high energies is used because, with the increasing energy, the absorption in the CsI decreases. The scintillator layer is configured thicker in order to absorb the majority of the X-ray or gamma photons. Typical layer thicknesses may be between 500 μm and 2000 μm. At these layer thicknesses, the spatial resolving power is consequently significantly reduced due to the light photons emerging laterally from the microstructures.
In order to counteract this effect, as described in the published application DE 10242006 A1, it has been attempted to structure the scintillator layer in a targeted manner. As mentioned above, however, from a thickness of 300 μm, said scintillator layer tends to grow together again, so that at greater layer thicknesses, the desired effect cannot be achieved.
In published applications DE 4433132 A1 and DE 10116803 A1, there are described attempts to color the surface of the microstructures in order to absorb light photons emerging laterally from the microstructure. However, the coloration is not restricted to the surface, but also extends into the interior of the microstructures. A raised level of absorption of the light photons in the interior of the microstructure is also associated therewith, which leads to a reduction in the desired signal.
The publication “Structured CsI (Tl) Scintillators for X-Ray Imaging Applications” by V. V. Nagarkar, IEEE Transactions on Nuclear Science, vol. 45, No. 3, June 1998, describes covering the microstructures with an optically absorptive protective layer.
The patent specification EP 1 793 457 B1 describes covering the microstructures completely from the base to the tip with a light photon-absorbing covering material. A covering of this type has a refractive index close to that of caesium iodide and therefore restricts the total reflection to relatively flat incident angles to the border layer between the caesium iodide and the covering material, such that more light photons enter the covering material and are absorbed there.
Protective layers made of parylene are known from the patent document U.S. Pat. No. 4,123,308 and the published application DE 19509438 A1.